HttpEntityMethodProcessor lets entity headers override existing headers (again)

Issue: SPR-15952

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessor.java

@@ -181,15 +181,15 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 
 		HttpHeaders outputHeaders = outputMessage.getHeaders();
 		HttpHeaders entityHeaders = responseEntity.getHeaders();
-		if (outputHeaders.containsKey(HttpHeaders.VARY) && entityHeaders.containsKey(HttpHeaders.VARY)) {
-			List<String> values = getVaryRequestHeadersToAdd(outputHeaders, entityHeaders);
-			if (!values.isEmpty()) {
-				outputHeaders.setVary(values);
-			}
-		}
 		if (!entityHeaders.isEmpty()) {
 			entityHeaders.forEach((key, value) -> {
-				if (!outputHeaders.containsKey(key)) {
+				if (HttpHeaders.VARY.equals(key) && outputHeaders.containsKey(HttpHeaders.VARY)) {
+					List<String> values = getVaryRequestHeadersToAdd(outputHeaders, entityHeaders);
+					if (!values.isEmpty()) {
+						outputHeaders.setVary(values);
+					}
+				}
+				else {
 					outputHeaders.put(key, value);
 				}
 			});

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessorMockTests.java

@@ -542,6 +542,59 @@ public class HttpEntityMethodProcessorMockTests {
 		assertConditionalResponse(HttpStatus.OK, "body", etagValue, -1);
 	}
 
+	@Test
+	public void varyHeader() throws Exception {
+		String[] entityValues = {"Accept-Language", "User-Agent"};
+		String[] existingValues = {};
+		String[] expected = {"Accept-Language, User-Agent"};
+		testVaryHeader(entityValues, existingValues, expected);
+	}
+
+	@Test
+	public void varyHeaderWithExistingWildcard() throws Exception {
+		String[] entityValues = {"Accept-Language"};
+		String[] existingValues = {"*"};
+		String[] expected = {"*"};
+		testVaryHeader(entityValues, existingValues, expected);
+	}
+
+	@Test
+	public void varyHeaderWithExistingCommaValues() throws Exception {
+		String[] entityValues = {"Accept-Language", "User-Agent"};
+		String[] existingValues = {"Accept-Encoding", "Accept-Language"};
+		String[] expected = {"Accept-Encoding", "Accept-Language", "User-Agent"};
+		testVaryHeader(entityValues, existingValues, expected);
+	}
+
+	@Test
+	public void varyHeaderWithExistingCommaSeparatedValues() throws Exception {
+		String[] entityValues = {"Accept-Language", "User-Agent"};
+		String[] existingValues = {"Accept-Encoding, Accept-Language"};
+		String[] expected = {"Accept-Encoding, Accept-Language", "User-Agent"};
+		testVaryHeader(entityValues, existingValues, expected);
+	}
+
+	@Test
+	public void handleReturnValueVaryHeader() throws Exception {
+		String[] entityValues = {"Accept-Language", "User-Agent"};
+		String[] existingValues = {"Accept-Encoding, Accept-Language"};
+		String[] expected = {"Accept-Encoding, Accept-Language", "User-Agent"};
+		testVaryHeader(entityValues, existingValues, expected);
+	}
+
+
+	private void testVaryHeader(String[] entityValues, String[] existingValues, String[] expected) throws Exception {
+		ResponseEntity<String> returnValue = ResponseEntity.ok().varyBy(entityValues).body("Foo");
+		for (String value : existingValues) {
+			servletResponse.addHeader("Vary", value);
+		}
+		initStringMessageConversion(MediaType.TEXT_PLAIN);
+		processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest);
+
+		assertTrue(mavContainer.isRequestHandled());
+		assertEquals(Arrays.asList(expected), servletResponse.getHeaders("Vary"));
+		verify(stringHttpMessageConverter).write(eq("Foo"), eq(MediaType.TEXT_PLAIN), isA(HttpOutputMessage.class));
+	}
 
 	private void initStringMessageConversion(MediaType accepted) {
 		given(stringHttpMessageConverter.canWrite(String.class, null)).willReturn(true);
@@ -554,8 +607,7 @@ public class HttpEntityMethodProcessorMockTests {
 		verify(stringHttpMessageConverter).write(eq(body), eq(MediaType.TEXT_PLAIN), outputMessage.capture());
 	}
 
-	private void assertConditionalResponse(HttpStatus status, String body,
+	private void assertConditionalResponse(HttpStatus status, String body, String etag, long lastModified) throws Exception {
-			String etag, long lastModified) throws Exception {
 		assertEquals(status.value(), servletResponse.getStatus());
 		assertTrue(mavContainer.isRequestHandled());
 		if (body != null) {